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Email Protocols Explained

Although it may not seem like it today – in the era of Smartphones, Tablets, and Facebook – I think we can all agree that email’s a pretty revolutionary technology.  The development of email rendered a whole crop of traditional communications methods all but obsolete.

But why does this matter?

Even though it’s been around for a while, it’s still every bit as relevant as it used to be. Understanding how email protocols work allows you to develop an email setup that will make your communications more efficient.

The best part?

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Receiving Email: IMAP vs. POP3


What Is IMAP?

IMAP(Internet Message Access Protocol) is the protocol used by the vast majority of consumer email accounts. With IMAP, emails are stored on an Internet server. When a user accesses their account, they’re connected to the external server, and data is transferred from the server to their local machine – which itself doesn’t actually receive any email messages.

One of the key advantages of IMAP – aside from remote access – is that it allows a single account to be operated and managed by multiple users, without the need for a complex workaround. It can also put a fair amount of strain on your mail server, if your business manages its own email service.

In order for IMAP to function, it requires one of two ports to be open:

  • Port 143: The default IMAP port; this allows the protocol to listen for incoming requests to synchronize emails. Traffic through this port is unencrypted.
  • Port 993: Used for IMAP over SSL.

What Is POP3?

Unlike IMAP, POP3 (Post Office Protocol 3) downloads messages from a mail service to a user’s local computer. This allows the user to not only receive emails on their own machine, but also to disconnect from the Internet and retain access to all of their messages. There are a number of advantages to this approach.

First and foremost, unless you’re opting to leave a copy of each email on the mail server, it clears up a significant amount of space by storing all files on the user’s local machine. This makes it much easier to back up one’s email messages, in addition to making search and organization far more efficient.

Unfortunately, POP3 isn’t without its weaknesses either. The protocol by default assumes that only a single user will be accessing a particular email account, meaning that you’ll need a workaround if you want multiple users on a single email. In addition, remote access can be something of a problem. Finally, the fact that most POP services delete downloaded emails from the server by default makes it incredibly difficult to synchronize an account to multiple machines.

POP3 requires one of the following two ports to be open:

  • Port 110: For unencrypted POP3 access; this is the default listening/communications port.
  • Port 995: Used for encrypted POP3 connections.

Which One Should You Use?

And now for the million dollar question…which of the two protocols should your business use?

The short answer is IMAP. Given the wealth of different devices typically used by modern employees, the capacity to remotely access one’s email regardless of location is absolutely essential.  The longer answer is that – as always – it depends entirely on what you need email for.

The truth is, there’s really nothing stopping you from using multiple protocols to access an email account – the technology does exist to do so, after all.

Sending Email: SMTP And HTTP


What Is SMTP?

Now, though there are two protocols associated with receiving emails, there’s really only one associated with sending them. SMTP(Simple Mail Transfer Protocol) is the gold standard for passing email messages from one mail server to another. Every email service uses some variation of this protocol.

There’s one more difference between SMTP and IMAP/POP3: unlike the latter two, it requires no authentication to function. Although modern SMTP applications restrict relaying, this nevertheless allows for a fair volume of spam to exist on the web; some tech-savvy criminals can even send emails using another’s account.

There are two ports you’ll need to be aware of for SMTP:

  • Port 25: This is the default SMTP port. It is not encrypted.
  • Port 465 / 587: The default port for using SMTP through SSL.

In addition, some hosts may offer a secondary port as an alternative to clients for whom port 25 is filtered.  

What Does HTTP Have To Do With Emails?

Now, granted, I’m certain a few of you are at least a little surprised to see HTTP (Hyper Text Transfer Protocol) included on this list. After all, it’s not really involved in the process of sending or receiving an email, nor does it directly interact with any of the other mail protocols. So…what does it have to do with email, exactly?

Easy – HTTP (and its more secure cousin, HTTPS) is the means by which you’re probably going to access your mailbox, particularly if you’re using a web-based email service like Hotmail or Gmail. For that reason, it actually occupies a fairly important position in the grand scheme of things, even if it’s not technically a mail protocol itself.

The Life-Cycle Of An Email: A Step-By-Step Process

Alright. Now that we’ve explained the protocols behind email, let’s take a look at how they actually come into play. The process begins when you send an email – the message is sent to a mail server tasked with the transport of emails.

This server is known as the Mail Transport Agent. The sender’s MTA proceeds to make use of the SMTP protocol to pass on the message to the recipient’s MTA, which itself proceeds to pass on that message to its Mail Delivery Agent. Once the MDA receives the email, it waits for the user to accept it, which is done either through POP3 or IMAP.

That’s…a bit confusing, isn’t it?

Let’s simplify things a bit. Perhaps the best analogy I’ve heard in regards to how this whole process functions comes courtesy of Kioskea, which likens MTAs to post offices and MDAs to mailboxes. Makes sense, right?

In Closing: A Few Additional Details You Should Know About

Now, it’s worth mentioning that there exists a myriad of variations on the POP3 protocol (and by association the IMAP protocol), as well as a number of proprietary email services such as Microsoft’s MAPI.  We haven’t mentioned any of those here today because they aren’t really necessary for an understanding of how email protocols function and how mail servers do their job.

Still, it’s good to be aware of them. Gotta know what your choices are, right?

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Andrew Horton

Andrew Horton

Andrew Horton is Digital Media Producer for ServerMania.

  • Great article. However, we are quite surprised that you did not mention Port 587. Port 587 is normally used for clients connecting to their ISP email server, and it is effectively SMTP+TLS. In this day and age, security is an absolute must, and while port 25 is still required for server to server communication, all email clients/customers should no longer be using straight POP(110),IMAP(143) or SMTP(25), but use the ‘encrypted’ versions, like IMAP-SSL and SMTP-TLS. This will ensure that their passwords aren’t ‘sniffed’ when sending/receiving email at their local coffee shop, or airport terminal. Also, Port 25 is routinely blocked at many of those locations, so using Port 587 will enable you to still use your favourite email client even at those locations.

  • It is interesting to note that we actually end up using different protocols to send and receive an e-mail. I always had this wrong impression that both send and receive operations can be done by same protocol.